This invention relates to the use of liquid metals as an indirect heat exchange fluid to indirectly heat or cool fluids containing high concentrations of hydrogen.
Liquid metal heat exchange systems have been generally used in the nuclear industry where characteristically high temperature differences between the heat source and the cooling medium require the use of a heat transfer fluid which will not change phase at any point in the heat exchange circuit. Such systems have been primarily used as a cooling medium in fast breeder nuclear reactors wherein heat build-up on the reactor side of the heat exchange equipment can occur quickly and it is required to remove this heat with a heat transfer fluid which remains a liquid over the full range of temperature.
In many industries, like the petrochemical and chemical industries, contact of reaction fluids with a catalyst in a reactor under suitable temperature and pressure conditions effects a reaction between the components of one or more reactants in the fluids. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things: poor product production, deactivation of the catalyst, production of unwanted by-products and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
Many arrangements seek to overcome the negative effects of endothermic chilling by supplying heat to the reaction or of exothermic heating by removing heat from the reaction. More traditional methods employ multiple stages of heating between adiabatic reaction stages. Other methods use in-situ heating via simultaneous reactions or indirect heat exchange to maintain an isothermal or other temperature profile within the reaction zone. U.S. Pat. No. 5,525,311 provides an example of indirect heat exchange with a heat exchange fluid to control the temperature profile within a reaction zone.
A variety of processes can employ indirect heat exchange within a reaction zone to control temperature profiles within the reaction zone. Common examples of hydrocarbon conversion reactions include the aromatization of hydrocarbons, the reforming of hydrocarbons, the dehydrogenation of hydrocarbons, the oxidation of hydrocarbons and the alkylation of hydrocarbons. Most of these hydrocarbon conversion processes process streams having high concentrations of hydrogen.
It is known to accomplish indirect heat exchange for processes with a variety of heat exchanger configurations including shell and tube heat exchange designs or thin plates that define reaction and heat exchange channels. In such arrangements the tubes typically contain catalyst while the channels contain a heat exchange fluid or in a plate arrangement the channels alternately retain catalyst and reactants in one set of channels and a heat transfer fluid in adjacent channels. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. A specific arrangement for heat transfer and reactant channels that offers more complete temperature control can again be found in U.S. Pat. No. 5,525,311, the contents of which are hereby incorporated by reference. Other useful plate arrangements for indirect heat transfer are disclosed in U.S. Pat. No. 5,130,106 and U.S. Pat. No. 5,405,586.
High heat capacity heat transfer fluids are used in several industries to provide cooling for shell and tube heat exchanger arrangements. Various types of high heat capacity fluids include alkali liquid metals such as sodium, lithium, and potassium and include molten salts such as nitrates and carbonates. These heat transfer fluid combine high heat capacity with high thermal conductivity. GB-2170898 generally discloses the use of sodium as a heat transfer medium in high temperature reactions including heat recovery from furnace installations, high pressure nuclear reactors, coal gasification, coal conversion, and water disassociation. U.S. Pat. No. 4,549,032 discloses the use of molten salt as an indirect heat transfer medium with a dehydration of styrene. German patent DE 2028297 discloses the use of an alkaline metal as a heat transfer medium in a process for producing alkenes and aromatics by cracking aliphatic hydrocarbons. The liquid metals are specifically used due to their high heat transfer capacity that permits utilization of small heating surfaces.
When indirectly heating or cooling hydrocarbons or other chemical feeds, the presence of hydrogen poses special problems for the use of liquid metals and other high heat capacity heat transfer fluids. Any finite hydrogen activity requires some provision for removal of metal hydride that will form from hydrogen that constantly permeates through the walls of the barrier between the fluids. Should the metal hydride concentration exceed solubility limits, the precipitation of solid hydride can interfere with the operation of the process or cause damage to equipment. Where the hydrogen permeation rate is small a chemical sorbent or getter material is used to chemically react and bind the hydrogen to prevent saturation of the metal hydride and its subsequent precipitation into the circulating system. Also the nuclear industry has used cold traps for many years to removal small quantities of sodium hydride.
Many hydrocarbon and petrochemical processes have a much higher hydrogen partial pressure on the process side of the heat exchange surfaces than the usual processes in which liquid sodium and other heat transfer fluids have been used. In many hydrocarbon conversion processes, the problem of hydrogen permeation can be severe. Many such processes work best with a relatively high hydrogen partial pressure which directly influences the problem of hydrogen permeation. Furthermore, obtaining a highly efficient heat exchange benefits from an increase in the surface area for the indirect heat exchange. As a surface area increases relative to the flowing fluid volume, the permeation of hydrogen into the liquid metal also increases. The recent trend in heat exchange arrangements for hydrocarbon conversion processes is to use a series of thin stacked plates which maximizes surface area, but at the same time, greatly increases the hydrogen permeation rate, particularly for those processes that maintain a relatively high hydrogen-to-hydrocarbon ratio. When hydrogen migrates into the liquid metal heat exchange fluid, the hydrogen forms the compound, sodium hydride. At low concentrations, the sodium hydride is soluble in the liquid metal heat exchange fluid, however, as the concentration of sodium hydride reaches a critical level, the sodium hydride begins to form a solid participate on the walls of the heat exchange surface, reducing the efficiency of the heat exchange process and if allowed to continue actually stopping the process.
Therefore, it is particularly desirable to have a process that can simply and effectively detect the concentration of hydrides in the liquid metal heat exchange fluid before the concentration of the hydrides reaches a critical level. Prior attempts to monitor the hydride level in liquid metal heat exchange systems employed very elaborate detection systems operating at very low vacuum conditions. For example, U.S. Pat. No. 4,403,500 discloses an apparatus for measuring hydrogen leaks in a liquid sodium heat exchange system in a steam generator of a fast breeder nuclear reactor. The device includes a technique for employing a single vacuum pump and a selector to maintain one or more measuring chambers at ultra high vacuum conditions and employing signals emitted by a mass spectrometer processed in a calculator unit to determine the equivalent hydrogen partial pressure in each of the chambers. The pressure in the chamber is maintained at about 10xe2x88x927 to 10xe2x88x928 torr. The hydrogen pressure is determined by Sievert""s law and the presence of hydrogen is used to indicate the presence of a water leak in the cooling coil of a nuclear reactor.
U.S. Pat. No. 3,731,523 discloses the use of a hollow nickel probe which is evacuated by an ion pump to a pressure of 10xe2x88x926 torr to measure hydrogen concentrations in sodium-cooled nuclear reactors. Nuclear reactors will have a normal hydrogen concentration level in the range of 0.1 to 2.0 ppm. Accordingly, the ion pump is turned off and the probe is inserted into a liquid sodium conduit. Hydrogen diffuses through the nickel and is allowed to reach an equilibrium pressure inside the probe. The pressure change in the probe is measured and Sievert""s law is applied to determine the hydrogen concentration in the sodium metal stream and to indicate the presence of a water leak in the cooling coil of a nuclear reactor. The ion pump is employed to re-evacuate the nickel probe for each measurement to detect hydrogen concentration of at least 0.1xc2x10.01 ppm. When such methods are employed to determine water leaks, they must be extremely sensitive to very small hydrogen concentration changes and detect such changes rapidly.
Typical cold traps that remove hydride precipitate or getters would quickly reach their capacity limit with the high hydrogen permeation rates associated with the chemical process. Replacement of cold traps and getter material will be prohibitively costly and inconvenient.
Those skilled in the art of using liquid metals as indirect heat exchange materials have addressed the problem of eliminating impurities, in particular, hydrides from the liquid metal streams. U.S. Pat. No. 4,713,214 shows a degassing chamber for purifying liquid metal coolant from a fast neutron nuclear reactor using a filter element that provides the primary means of purification and a degassing chamber that collects bubbles of an inert gas blanket that may become entrained in the circulating liquid sodium. U.S. Pat. No. 4,581,200 uses a tank in combination with a cold trap wherein the tank deposits a sodium mist in contact with hydrogen to act as a hydrogen getter for subsequent intermediate release of hydrogen by heating of the sodium deposit. U.S. Pat. No. 4,290,822 discloses a method for cleaning a cold trap that uses sodium hydroxide to transform heated impurities into liquid phase and then draining off the liquid phase that may use vacuum conditions to remove any possible traces of water. U.S. Pat. No. 3,941,586 also teaches the purification of cold trap by heating sodium hydride to a molten state and removing or venting hydrogen gas from the cold trap. The typical apparatus associated with a cold trap comprises an economizer exchanger that transfers heat between hot, unpurified metal and the cold purified metal, a cooler for the liquid metal, and some form of retainer for a filtering element, or metallic fibers. It is known from U.S. Pat. No. 4,713,214 that cold trap devices may be integrated in a reaction vessel or may be external to the reaction vessel and involve a secondary circulation loop. U.S. Pat. No. 4,290,822 discloses the heating of cold traps with resistance heaters to maintain a temperature of about 355xc2x0 C. to dissolve sodium hydride and sodium hydroxide. None of these methods are particularly suited for process fluids that have a high hydrogen concentration.
It is, therefore, an object of this invention to provide a method of detecting hydride from circulating liquid metal heat exchange fluids that can accommodate a high hydrogen permeation from the process fluid.
It is a further object of this invention to provide a simplified system of detecting and providing advisory control for removing hydrogen and the resulting metal hydride from a circulating liquid metal heat transfer fluid that facilitates the regeneration of traps for further purification of the circulating liquid metal material.
This invention is a process for the use of a hydrogen sensor to be used with liquid metal heat exchange fluids in a process that has a high hydrogen permeation. It was surprisingly discovered that at the high levels of hydrogen which migrate into a liquid metal heat exchange loop in a hydrocarbon conversion process, such processes can employ a simplified hydrogen sensor to effectively report the high hydrogen partial pressure levels in the liquid metal stream without the need to re-evacuate the sensor between determinations. Furthermore, it was found that the rate of change of the hydrogen partial pressure was sufficiently rapid enough to employ the hydrogen partial pressure reported to manage the liquid metal stream between heat exchange service and regeneration for hydrogen removal.
Accordingly, in one embodiment, the present invention is a process for using a hydrogen sensor in a hydrocarbon reaction zone. The hydrocarbon reaction zone operates under hydrogen partial pressure while indirectly exchanging heat with a liquid metal stream that circulates in a heat exchange loop. A portion of the hydrogen from the reaction zone migrates across a heat exchange surface into a recirculating liquid metal stream in the heat exchange loop. At least a portion of the recirculating liquid metal stream is passed to a hydrogen removal zone and returned to the recirculating liquid metal stream. Intimate contact with at least a portion of the recirculating metal stream and a hollow nickel membrane probe of a hydrogen sensor is provided. The hydrogen sensor comprises: a hollow cylinder containing the hollow nickel membrane probe; a vacuum chamber in fluid communication with the hollow nickel membrane probe; and a pressure transducer, establishing equilibrium between the hydrogen that passes through the nickel membrane probe to the vacuum chamber and the hydrogen in the recirculating liquid metal stream. The hydrogen sensor is operated at effective sensor conditions to measure hydrogen partial pressure in the vacuum chamber over a hydrogen partial pressure range of about 2 to about 10 mm Hg.